In-situ activation of self-supported 3D hierarchically porous Ni3S2 films grown on nanoporous copper as excellent pH-universal electrocatalysts for hydrogen evolution reaction

The exploitation of low-cost, stable, and highly active electrocatalysts based on earth-abundant metals for hydrogen evolution reaction (HER) is crucial for developing renewable energy techniques. In this work, we report a facile synthesis strategy for in-situ fabrication of 3D hierarchically porous Ni 3 S 2 films on a nanoporous copper substrate (Ni 3 S 2 @NPC) by unusual galvanic replacement reaction in the Ethaline-based deep eutectic solvent (DES) under a normal atmosphere. The self-supported nanoporous Ni 3 S 2 @NPC electrode is binder- free and exhibits good structural integrity with high conductivity. A mild evolution of bulk gas bubbles (H 2 -O 2 gas mixture) is proved to drive an in-situ structure rearrangement process of the Ni 3 S 2 @NPC and results in substantial increases in the HER activity. The activated Ni 3 S 2 @NPC (a-Ni 3 S 2 @NPC) electrode can serve as a highly efficient and stable electrocatalyst for the HER in water over a wide pH range. Significantly, it displays high-performance HER catalytic activity in acidic media with robust durability over 111   h and functions well under alkaline and neutral conditions. Such a superior catalytic performance of the a-Ni 3 S 2 @NPC is mainly due to the unique hierarchically nanoporous architectures and the synergetic effects in it caused by the restructuring NPC skeletons and active components. Our work offers a generic strategy for design and fabrication of many other self-supported transition metal sulfide and phosphide based HER electrocatalysts, and uncovers a new O 2 -induced electrochemical self-activation mechanism for improving the activity of catalysts.